To improve the shear resistant of hot melt adhesives at elevated temperatures, reactive adhesive systems are used which are either cross-linked by supplying energy or cured by means of moisture to form an infusible adhesive.
Reactive, moisture-curable hot melt adhesives of the isocyanate-functional polymer type are known, for example from German Offenlegungsschrift 2,401,320, Published European Application 0 107 097 and Published European Application 0 125 009. The polymers are predominantly polyesters of adipic acid, 1,4-butanediol and 1,6-hexanediol.
For many applications, it is necessary that the reactive hot melt adhesives set rapidly after application, so as to make immediate further processing possible. However, this is difficult to achieve with known hot melt adhesives. An improved, that is, a shorter setting time requires additives such as resins or thermoplastic polymers. Liquid isocyanate prepolymers are combined according to Published European Application 0 232 055 with ethylene/vinyl acetate copolymers or ethylene/methylstyrene resins, according to Published European Application 0 107 097 with thermoplastic polyurethanes or condensation resins, and according to Published European Application 0 246 473 with acrylate oligomers. After being cross-linked by the humidity of the air, such reactive hot melt adhesive still contain high proportions of thermoplastic resins, which disadvantageously affect the shear resistance at elevated temperatures.
While hot melt adhesives disclosed in Published European Application 0 248 658, which contain polyesters with more than 50% aromatic dicarboxylic acid instead of aliphatic dicarboxylic acid, have an improved setting rate, such products have the disadvantage of a melt viscosity that is too high. This causes problems during the preparation of the prepolymers and while processing the hot melt adhesives. Moreover, because they contain aromatic dicarboxylic acids, the glass transition temperature of these hot melt adhesives is increased substantially, which leads to a lower elasticity of the adhesive bond after cross-linking.